A water-absorbent resin has been hitherto used as one component for hygienic materials such as sanitary cotton, disposable diaper, and hygienic goods of absorbing another body fluid. As an example of such a water-absorbent resin, a hydrolysate of a starch-acrylonitrile graft polymer, a neutralized starch-acrylic acid graft polymer, a saponified vinyl acetate-acrylic acid ester copolymer, a hydrolysate of an acrylonitrile copolymer or acrylamide copolymer, a crosslinked substance thereof, and partially neutralized crosslinked acrylic acid or the like are cited, for example. These water-absorbent resins invariably possess an internal crosslinked structure and exhibit no solubility in water.
Characteristic properties, which these water-absorbent resins are expected to possess, include high absorbency, excellent absorption speed, high gel strength, and excellent suction force necessary for sucking water from a substrate and the like. Because water-absorbing properties are affected by crosslink density, they do not necessarily show positive correlations with one another, as evinced by the fact that increase in crosslink density leads to increase in gel strength but decrease in the amount of water absorbed. Particularly, absorption capacity is in a contradictory relation with absorption speed, gel strength, and suction force and the like. Therefore, in the water-absorbent resin particle, which has acquired enhanced absorbency, when the water-absorbent resin particles contact with water, water is not diffused throughout the entire volumes of water-absorbent resin particles, to induce extreme deterioration of absorption speed, since uniform absorption of water is not carried out and forms portions of partial aggregation of itself.
For the purpose of relaxing this phenomenon and obtaining the water-absorbent resin having high absorption capacity and relatively satisfactory absorption speed, a method for coating a surface of the water-absorbent resin particles with a surfactant or a nonvolatile hydrocarbon has been known. This method indeed improves dispersibility of initially absorbed water but brings no sufficient effects in enhancing absorption speed and suction force of the individual resin particle.
In addition, as a method for producing a polyacrylic acid-type polymer with improved water-absorbing property, there has been proposed a method for subjecting an aqueous composition having a partial alkali metal salt of polyacrylic acid as a main component and having low crosslink density, to heating in the presence of a water-soluble peroxide radical initiator, thereby introducing a crosslink therein by radical crosslinking (U.S. Pat. No. 4,910,250). It is difficult to distribute uniformly internal crosslinks in the polymer and uneasy to adjust the crosslink density. Thus, there has been adopted a method for preparing a polymer which contains water-soluble polyacrylic acid gel having low crosslink density, and then heating the polymer together with a persulfate as a polymerization initiator added thereto. U.S. Pat. No. 4,910,250 has disclosed that the precise control of crosslink density is realized by adjusting the added amount of the initiator and, due to the uniform presence of crosslinks in the polymer, excellent water-absorbing characteristics is obtained and the water-absorbent resin having no stickiness is obtained.
While the persulfate, which is used in U.S. Pat. No. 4,910,250, is decomposed by heat, it is also decomposed by ultraviolet rays to generate radicals (J. Phys. Chem., 1975, 79, 2693, J. Photochem. Photobiol., A, 1988, 44, 243). Because persulfate function as a polymerization initiator, an aqueous solution of a water-soluble vinyl monomer, when exposed to irradiation, dissociates the initiator to generate radicals, and a monomer radical generated attacks the next monomer to enable production of synthetic polymer gel (EP-A-1 400 538). In the method in EP-A-1 400 538, polymerization of a water soluble vinyl monomer and crosslinking of the generated polymer can be carried out simultaneously, by irradiation of UV rays to an aqueous solution containing the water soluble vinyl monomer having a specific structure and a persulfate salt. It should be noted that there is a reaction system which forms an internal crosslink by adding a hydrophilic polymer component, a photo-polymerization initiator, and still more a crosslinking agent together and irradiating them with light (US-A-2006-052,478). In US-A-2006-052,478, crosslinked hydrogel is obtained by using a persulfate salt as a photo-polymerization initiator, and radiating UV rays to an aqueous solution of a water soluble polymer in the presence of a crosslinking agent.
Meanwhile, there has been also known a method for enhancing crosslink density of the surface of a water-absorbent resin by treatment of the surface of the water-absorbent resin with a crosslinking agent (for example, U.S. Pat. No. 4,666,983 and U.S. Pat. No. 5,422,405). As described above, the water-absorbent resin is a water-insoluble polymer having internal crosslink, which is produced by formulating an internal crosslinking agent and a polymerization initiator to a polymerizable monomer and polymerizing. On the surface of the water-absorbent resin after polymerization, reactive functional groups contained in the monomer are present. Therefore, by introduction of a crosslink between functional groups by the addition of a surface crosslinking agent capable of reacting with the functional groups, it is possible to provide a water-absorbent resin having increased crosslink density and excellent water-absorbing characteristics even under pressure.
However, in the case of using the above surface crosslinking agent, there are problems in that a crosslink formation reaction must be performed at high temperature for a long period of time, and a crosslinking agent remains unreacted or the like. In consideration of this, there has been proposed also a method for introducing crosslinks to polymer molecular chains at the vicinity of the surface of the resin by decomposition of a radical initiator, by subjecting an aqueous solution containing a peroxide radical initiator to contact with a resin, and heating the resin (U.S. Pat. No. 4,783,510). In examples of U.S. Pat. No. 4,783,510, the water-absorbent resin exhibiting excellent absorption capacity is obtained by heating with superheated steam at 130° C. for 6 minutes. In addition, there has been disclosed a method that the water-absorbent resin of enhanced water absorption capacity is obtained by impregnating a hydrophilic multifunctional unsaturated compound such as N,N′-methylenebisacrylamide into an aqueous solution containing persulfate as a radical initiator, and contacting the aqueous solution with the resin, and then heating.
In addition, there has been proposed also a method for producing an improved water-absorbent resin having higher crosslink density at the vicinity of the particle surface of the water-absorbent resin as compared with the inside of the particle, by impregnation, polymerization and heating a water-soluble ethylenically unsaturated monomer into a water-absorbent resin (JP-B-2,530,668). In this case, the water-absorbent resin having higher crosslink density at the vicinity of the surface as compared with the inside of the particle is obtained, because a crosslinking agent is introduced into a solution containing the water-soluble ethylenically unsaturated monomer in higher concentration than concentration of the crosslinking agent used in polymerization of the water-absorbent resin, to form a polymerization layer on the surface of the water-absorbent resin, by using this solution. In the method described in JP-B-2,530,668, there can be used a water-soluble radical polymerization initiator such as potassium persulfate, sodium persulfate in surface treatment, and in the Example, polymerization is initiated by heating this at 60° C.
As other technology aiming at improving various water-absorbing characteristics of the water-absorbent resin, there have been proposed methods for enhancing crosslink density at the vicinity of the surface of the water-absorbent resin, by surface treatment of the water-absorbent resin (JP-A-63-99,211, JP-A-1-126,314, JP-A-2007-119,757, JP-A-2005-97,585, WO 2006/62,253, JP-A-2005-213,523).